792 PRINCIPLES OF SOIL MICROBIOLOGY 



upon the soil, as pointed out elsewhere (p. 743). Magnesium car- 

 bonate becomes toxic in large amounts, although in small quantities 

 it may be more effective than calcium carbonate. 100 



Some microorganisms do not grow in the soil when the acidity is 

 greater than a certain maximum. Of particular interest in this con- 

 nection is the occurrence and development of Azotobacter in the soil. 

 The presence of this organism can be used as an index of the lime 

 requirement of the particular soil. 101 When the reaction of the soil 

 is less than pH 5.7 (no Azotobacter present), the soil needs lime; 

 when the pH is above 7.4, the soil does not need any lime. But when 

 the pH of the soil is between 5.8 and 7.3, an Azotobacter test is made. 

 A definite weight of soil (5 or 10 grams) is added to a definite amount 

 of mannite solution, free from calcium carbonate (50 or 100 cc), and the 

 flasks are sterilized ; these are then inoculated with a fresh culture of Azo- 

 tobacter and incubated. The amount of pellicle development is an 

 index of the buffer action of the soil and can yield information on its 

 lime requirement. When lime is added to an acid soil in which Azo- 

 tobacter is lacking, it will not take very long before this organism will 

 appear. When a culture of Azotobacter is added to soils in which it 

 is lacking or not abundant, it will survive only in those soils in which the 

 reaction is above pH 6.0. The whole question of influence of reaction 

 upon the growth of microorganisms is discussed in detail elsewhere 

 (p. 637). 



Influence of growing plants upon soil microorganisms and their activities. 

 The growing plants exert various influences upon the activities of the 

 microorganisms in the soil: 



1. They excrete soluble substances which offer a favorable medium 

 for the activities of microorganisms. 102 Under sterile conditions, 



formation on the influence of calcium upon the activities of microorganisms is 

 given by Miller. Uber den Einflusz des Kalkes auf Bodenbakterien. Ztschr. 

 Giirungsphysiol., 4: 194. 1914; Plummer, J. H. The effects of liming on the 

 availability of soil potassium, phosphorus and sulfur. Jour. Amer. Soc. Agron., 

 13: 162-171. 1921. 



100 Lipman, J. G., and Brown, P. E. N. J. Agr. Exp. Sta. 28th Ann. Rpt., 

 141-204, 1907. 



101 Christensen, H. R. Untersuchungen uber einige neuere Methoden zur 

 Bestimmung der Reaktion und des Kalkbedurfnisses des Erdbodens. Intern. 

 Mitt. Bodend., 13: 111-146. 1923. 



102 Wilson, J. K. The nature and reaction of water from hydathodes. Cornell 

 Univ. Agr. Exp. Sta. Mem., 65. 1923; also Lyon, T. L., and Wilson, J. K. Ibid. 

 Mem. 40. 1921. 



